1. Field of the Invention
The present invention concerns a metalization structure and, more in particular, it relates to a metalization structure suitable to a case of using a conductor layer of a conductive material comprising nickel as a main ingredient, as well as a manufacturing method thereof.
2. Description of the Related Art
In recent years, as development has been made for increasing the number of layers, increasing the degree of integration and improving the performance of wiring substrates for mounting LSI, higher characteristics have been demanded for insulative films used therefor. As one of such insulative films, polyimides have been used generally at present in view of their excellent thermal and chemical stability.
For mounting a wiring substrate such as a multichip module and electronic parts such as LSI by way of soldering connection, connecting electrodes are disposed to the wiring substrate. In this case, nickel or nickel-based alloy is desirably used for the outermost layer of the electrode (layer in direct contact with solder). This is because of facilitating repair of the soldering connection.
In view of the above, as a method of forming an electrode for soldering connection, there may be considered, as shown in FIG. 6, a method of providing an aligning pad 63 comprising three Cr/Cu/Cr layers on the surface of a wiring substrate 62 having copper wirings 61, forming an Ni--W alloy electrode 64 on the surface, insulating the same by covering with a polyimide film 65 and forming through holes 66 to the polyimide film. In this method, the entire surface of the electrode 64 facing the aligning pad 63 is in an intimate contact with the surface of the aligning pad 63 to obtain a metalization structure in which ends of the aligning pad 63 and the electrode 64 are covered with the polyimide insulative film 65.
This method has a merit capable of collectively making film formation and fabrication for the aligning pad and the electrode respectively.
When the electrode 64 is formed in this method, it is necessary to etch the Ni--W alloy film into a shape of the electrode with phosphoric acid+nitric acid+acetic acid system. However, since the etching solution also etches Cr and Cu, it involves a problem that the aligning pad 63 in the lower layer is eroded from the end upon etching of the electrode 64.
Further, since the film stress of Ni--W is large, when the electrode 64 is formed by etching, there is a problem that the conductor portion (the electrode 64 and the aligning pad 63) is cleaved or peeled from the substrate or the substrate suffers from cracking or chipping by the film stress. That is, when a pattern of a conductor mainly composed of a copper is formed on the underlying layer such as a glass substrate, ceramic substrate or polyimide film and a conductor pattern comprising nickel as a main ingredient is formed on the above-mentioned pattern, a strong stress is caused between the conductor comprising copper as a main ingredient and the conductor comprising nickel as a main ingredient to cause cracking or chipping in the underlying layer. The cracking or chipping hinders electric connection of internal conductors in the underlying layer or causes peeling of the conductor comprising nickel as a main ingredient from the underlying layer to remarkably deteriorate the reliability of the metalization structure.